Modern electronics, such as smart phones and personal digital assistants, are packing more integrated circuit functions into an ever-shrinking physical space with expectations for decreasing cost. In order to facilitate these requirements, several functions may reside within the device. These functions may include wireless internet access, a camera, address book, or a data storage and retrieval system. Each of these functions will share the same user interface controls for their operation. As the individual functions may supply different levels of interface signals an amplifier must provide a gain control path so that several functional sources may share the same user interface structure.
Historically the amplifier has been the primary signal amplitude management structure throughout the analog electronics world. There are several aspects of an amplifier that must be satisfied in order for it to meet the requirements of the design. The primary amplifier architecture used in analog integrated circuits is the differential amplifier. The differential amplifier is typically made up of a closely matched pair of transistors that are positioned very close together in the circuit and coupled in a common gate configuration. An additional pair of transistors may be used as input signal paths to the amplifier.
The gain (A) of an amplifier controls the amount of “boost” a signal is subjected to in the amplification process. The gain may be greater than 1 in order to grow a signal or less than 1 in order to shrink the signal to a proper interface level. The gain (A) may be represented in an equation by the transconductance (gmd) of the signal input transistors, M1 and M2, times the output impedance of M2 (1/gds2) in parallel with the output impedance of the differential transistor M4 that drives the output voltage Vout. This can be shown in equation 1 below.
                    A        =                              g            md                                              g                              ds                ⁢                                                                  ⁢                2                                      +                          g                              ds                ⁢                                                                  ⁢                4                                                                        (        1        )            
Often it is desirable to decrease the gain (A) of a stage. The gain (A) can be adjusted to a very limited extent by changing the geometries, as in the physical dimensions of the transistors in the circuit. The gain (A) may also be decreased bay adding a source resistor in each leg of the differential input. The resistors (R1 and R2) may be very large due to the small currents that are normally used in the amplifier circuits. If the resistors (R1 and R2) are too large it may not be practical to implement them because their physical size would surpass the size of the transistors in the amplifier.
A low gain amplifier circuit may be implemented by separating the gates of the differential pair transistors (M3 and M4) and individually coupling the gate to their own drain of the transistors. The gain, within a factor of 2, may be calculated by equation 2 below.
                    A        =                              g            md                                              g                              ds                ⁢                                                                  ⁢                2                                      +                          g                              m                ⁢                                                                  ⁢                4                                                                        (        2        )            
The transconductance (gm4) is much greater than gds2 or gds4 and therefore greatly reduces the gain. This circuit, however, only reduces the gain to a very low fixed value. The difficulty associated with adjusting the gain of the differential amplifier to some intermediate level has been very difficult for the designer and the manufacturing process.
Thus, a need still remains for an integrated circuit system for controlling amplifier gain. In view of the demand for an increased number of functions in the standard package formats, it is increasingly critical that answers be found to these problems. In view of the ever-increasing commercial competitive pressures, along with growing consumer expectations and the diminishing opportunities for meaningful product differentiation in the marketplace, it is critical that answers be found for these problems. Additionally, the need to save costs, improve efficiencies and performance, and meet competitive pressures, adds an even greater urgency to the critical necessity for finding answers to these problems.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.